This invention relates to a drum brake device. More specifically, it relates to a drum brake device that functions as a leading-trailing (LT) type when the service brake is applied, and as a duo-servo (DS) type when the parking brake is applied.
This type of drum brake device has been disclosed in Australian Patent No. AU-B1-53 491/79 and U.S. Pat. No. 5,275,260. The basic brake functions are the same in both devices, and the structure will be explained with reference to FIG. 17. A pair of brake shoes b, c are provided on top of a back plate a. An anchor block d is provided between the adjacent ends of one side of each brake shoe, and a hydraulic cylinder g is provided between the other adjacent ends of each brake shoe. A parking lever j is pivoted on one end i of one brake shoe b. An idler lever k is pivoted so as to swing on the other brake shoe c. The first and second rods l, m are provided between the two brake shoes b, c, wherein one end n of the first rod l engages the parking lever j, and the other end o engages with the idler lever k; and one end p of the second rod m engages one brake shoe b, while the other end q engages with the other brake shoe c and the idler lever k.
The brake action is explained as follows. When the driver steps on the brake pedal, the hydraulic cylinder g is pressurized, wherein the two brake shoes b, c spread open, with the point of abutment with the anchor block d as the fulcrum, to cause a frictional connection with the rotating brake drum, not shown in the diagram, in a leading-trailing braking action.
When the parking brake is applied, the parking lever j is pulled in the direction of the arrow X. The force of that action is transferred in sequence to the first rod l, idler lever k, and second rod m, wherein the one brake shoe b opens with its point of abutment with the anchor block d as the fulcrum, and to cause a frictional connection with the brake drum. Next, the idler lever k spreads open, with the point of abutment with the second rod as the fulcrum, causing the pivot component of the idler lever k to press the other brake shoe c in the direction of the arrow Y to cause a frictional connection with the brake drum. At the same time, the reaction force of the parking lever j is being applied in the direction of the arrow Z on the one end i of the one brake shoe b.
If at this time, the torque is applied on the brake drum in the direction of arrow R (uphill or downhill), the friction force of the one brake shoe b is transferred to the second rod m, wherein its other end q presses against the other brake shoe c, supported by the anchor block d, in a duo-servo braking action. If the torque is applied on the brake drum in the opposite direction of arrow R, the friction force of the other brake shoe c is transferred to the second rod m, whereby its one end p is pressed against the one brake shoe b, supported by the anchor block d, in the same duo-servo braking action as above.
As is evident from this parking brake operation, if the other end q of the second rod m abuts the idler lever k and a gap exists between the other end q and the other brake shoe c, then whether the shoe c rotates in the opposite direction of R, the piston of the hydraulic cylinder g is repelled to an amount equivalent to the gap. That is, the brake pedal is repelled which is not only disconcerting to the driver, but the pedal stroke increases in the next brake pedal application. Conversely, if the other end q of the second rod m abuts against the other brake shoe c and a gap exists between the other end q and the idler lever k, then the stroke of the parking lever j increases by an amount equivalent to this gap; that is, the stroke of the hand lever increases. From these perspectives, it is preferable that the gap between the other end q of the second rod m and either the other brake shoe c or the idler lever k be as small as possible.
Next, FIG. 18 illustrates the concept of the automatic shoe clearance device which is installed in the drum brake device of U.S. Pat. No. 5,275,260. The bent end y of the adjustment lever r is pivotable on the web of brake shoe c, and one upper arm s is connected to the groove of the upper strut t for their interaction. Another arm is connected to the star wheel u of the upper strut t. A spring w, stretched between the adjustment lever r and the pivot lever v, energizes the adjustment lever r in the counterclockwise direction, with the end y as the fulcrum.
Should the brake lining be worn causing the two brake shoes b, c to open by more than a prescribed value when the service brake is applied, the upper arm causes the star wheel u to rotate to automatically extend the entire length of the upper strut t, thereby maintaining a constant clearance between the brake shoes b, c and the brake drum z.
The drum brake device as described above has need of improvement in the following areas:
The cumulative effect of the tolerances of each component of the parking brake system will inevitably create a gap between the idler lever k and the first rod l or the second rod m, and the brake stroke becomes ineffective by an amount equivalent to this gap.
Moreover, as the lining of the other brake shoe c gradually wears, there is a gradual shift in the point at which the second rod m contacts with the brake shoe c or the idler lever k. That is, as shown in FIG. 17, the amount of displacement .delta. of the brake shoe c at the brake center, and the amount of displacement .delta.c and .delta.k of the brake shoe c and idler lever k respectively at the point of contact with the second rod m are defined as follows: ##EQU1## H1: Distance from anchor d to the brake center (pivot point of brake shoe c and idler lever k);
H2: Distance from the brake center to the second rod m; PA1 H3: Distance from the brake center to the first rod l; PA1 .delta.: Amount of displacement.
In this case, H3 is considerably smaller than H1, hence the displacement .delta.k of the idler lever k will be considerably larger than the displacement .delta.c of the brake shoe c. As a result, the stroke of the parking lever j will increase as the lining wears. The driver not only feels a greater slackness in the hand brake, but there are concerns as well that the lever j could interfere with other components to lessen the effectiveness of the braking action. In addition, the stroke of the parking lever could also limit the brake size, brake offset, and other brake factors to constrain the degree of freedom in the brake design.
If H3 (distance from the center of rotation of the idler lever k to the first rod l) and H2 (distance from the same point to the second rod m) are set to be equal, and the brake drum rotates in the direction of the arrow R when the parking brake is applied, then the one brake shoe b, the parking lever j and other components will turn in tandem in the same direction (the degree of rotation will be equivalent to the distance traveled by the other end q of the second rod until it starts to press against the other brake shoe c). FIG. 19 is a conceptual drawing of this operation. The double-dash broken line shows the state of the device if the parking brake is activated after the vehicle is stopped by the service brake on a slope. The one ends i of both brake shoes b, c are supported by the anchor block d. Then if the service brake is released in this state, the force of gravity acting on the vehicle causes the brake drum, not shown in the diagram, to rotate in the direction of the arrow R, whereby the one brake shoe b and the parking lever; are shifted to the positions indicated by the solid line and broken line respectively. As a result, if point s is the outlet of the parking brake pulling mechanism, then the distance from point s to the pulled part of the parking lever j is shortened by an amount equal to (l.sub.1 -l.sub.2). In other words, the pulled part of the parking lever j is shifted in the direction of the pull to loosen the locked parking brake pulling mechanism, thus reducing the effectiveness of the parking brake.
When the parking brake is applied, the adjacent ends of each side of the both brake shoes move apart to spread apart the whole brake assembly. In other words, the one end of both of the shoes also separate at once from the anchor block. When the brake drum which is interlocked with the wheels starts to rotate with the device in this state, both brake shoes, the rods, and other components all turn in tandem, wherein one or the other brake shoe collides against the anchor block. The noise so generated is not only disconcerting to the driver. Additionally, since this impact load is applied repeatedly on the anchor block, the strength of the components becomes critical.
In the conventional device as disclosed in Australian Patent No. AU-B1-53 491/79, the cumulative effect of the tolerances of each component will be such that the idler lever could abut against the second rod or play could be generated. At the very least, play will be generated when the driver steps on the brake pedal. Accordingly, the idler lever could vibrate creating a strange noise when the vehicle is in motion or the service brake is applied. Again this noise can be disconcerting to the driver.
In the conventional device as disclosed in U.S. Pat. No. 5,275,260 as the lining wears, the adjustment lever becomes interlocked with the brake shoe and moves with its point of abutment with the support block as the fulcrum, while the pivot lever moves with its point of abutment with the lower strut as the fulcrum. This changes the energizing force of the adjustment spring energizing the adjustment lever, which has a negative effect on the automatic adjustment process when only a very minimal adjustment is required.